Nodular graphite steel



March 7, 1961 TOHEI OTOTAN] 2,974,035

NODULAR GRAPHITE STEEL Filed June 10, 1958 F/EE.

United States Patent 1C Patented Mar. 7, 1961 'NODULAR GRAPHITE STEELTohei Ototani, Tokyo, Japan, assignor to The Research Institute forIron, Steel and Other Metals 01: the Tohoku University, Sendai City,Japan Filed June 10, 1958, Ser. No. 741,143 Claims priority, applicationJapan Oct. 12, 1957 3 Claims. 01. 75-123) i assumed that nodulargraphite is precipitated by considerably high atomic interaction at ahigh temperature between the atoms of the added elements such as cerium;calcium, magnesium and the like elements and the carbon atoms containedin the steel.

The accompanying illustration shows the microscopic structure of thesteel produced by this invention. In the illustration: I Fig. 1 is amicroscopic photograph showing a micro structure multiplied one hundredtimes of a hypo-eutectoid carbon steel (0.53% C, 0.74% Si, 0.28% Mn,0.008%

P, 0.008% S) treated with 1.0% of iron-calcium base alloy (22.7%Ca),.annealed at 900 C. for 3 hours after casting, and

Fig. 2 is a similar microscopic photograph enlarged.

400 times.

graphite structure having improved mechanical and physical properties.

A further object of this invention is to produce castings and ingots ofcarbon steels and of alloy steels having nodular graphite by treatingchemically the molten bath of such steels.

The principle of the present invention is based on pre cipitating thenodular graphite by adding to the hypereutectoid steel a small amount ofcalcium and/ or cerium together with a suitable flux if desired, therebychanging the primary free cementite into nodular graphite, in order toobtain a new material possessing the machining and wear-resistantproperties ascribed to the presence of graphite as well as theresponsiveness to various heat treatments and hot or cold-workability ofsteel.

Hyper-eutectoid steel contains primary cementite in a Accordingly, inorder to improve its mechanical properties-a method had been adopted tospheroidize or graphitize the cementite by suitable heat treatment.

It has now been found that the above precipitated free cementite can beavoided by adding a suitable amount of calcium and/ or cerium to themolten bath of the hypereutectoid steel, and that the precipitatedgraphite can be nodularized. It has also been found that besides calciumand cerium, a suitable amount of either one or more ele-' ments from thegroup consisting of magnesium, lithium,- strontium and barium may beadded to the molten bath of said steel to produce a nodular graphite inthe cast state. Calcium may be added in the form of Fe-Ca base alloycontaining 10 to 80% of iron, 5 to 40% of calcium, and 5 to 55% of oneor more of Ni, Si, Al or Mn or calcium silicide or any suitable calciumalloy. Usually 0.2 to 0.6% of Fe-Ca base alloy or 0.5 to 8% of calciumsilicide is employed together with 0 to 2% of a flux containing calciumfluoride or magnesium fluoride as the main ingredient.

As an alternative, the amount of calcium to be added may be regulated tomake the retained calcium in the casting of hyper-eutectoid steel lessthan 0.2%, and further cerium is added to maintain the remaining ceriumat 0.005 to 0.15%.

This invention can be applied equally to eutectoid or hypo-eutectoidsteels, i.e., to carbon steels containing less than 0.85% C and also tospecial alloy steels containing less than about 1.7% C together with oneor more elements selected from the group consisting of Mn, Cr, W,

M0, V, Ti and similar elements forming carbides.

The phenomenon of precipitating nodular graphite by this invention evenin eutectoid or hypo-eutectoid steels, in which primarily precipitatedcementite is not produced, is diflicult to illustrate diagrammatically,but it may be From Figs. 1 and 2, the distribution of precipitatednodular graphite in a matrix of pearlite and ferrite can be clearlyrecognized.

The standard structure of a common eutectoid steel consists mainly ofpearlite matrix only, while that of hypo-eutectoid steel consists offerrite and pearlite and is located onthe side of iron from theeutectoid point as seen in the constitutional diagram. It is found,however, that according to this invention, by adding the above Imentioned elements perfect nodular fine-grained graphite ispre'cipitatedeven in eutectoid and hypo-eutectoid steels. The accompanying drawingillustrates the nodular graphit estructure of hypo-eutectoid steel(about 0.6% C) obtained by this invention, showing that the graphiteexists 1 over-the whole area of pearlite and ferrite matrix; it hasprecipitated at a high temperature, before primary ferrite and pearlitehave been produced.

Steel means here an alloy consisting of iron and less than 1.7% ofcarbon as the main ingredients and containing in the matrix a smallquantity of silicon and pearlite matrix, and so it has high hardness butis brittle.

manganese as well as phosphor, sulfur, etc. as impurities. The steel tobe produced by this invention contains very little oxygen and sulfur,such as 0.001 to 0.005%

of oxygen and 0.005 to 0.015% of sulfur. 1

Further, in carrying out this invention, the molten bath to be treatedshould be previously de-oxidized and desulfurized as perfectly aspossible because this procedure is effective in retaining the abovedescribed addition elements in the product. That is, the above mentionedaddition-elements, cerium, calcium, lithium, strontium, barium andmagnesium are added to the molten steel, so that they react rapidly withthe oxygen and sulfur in the bath to form their oxides and sulfideswhich float upon the surface of the bath. Accordingly, when suchelements are to be added, the loss of the additional elements due todeoxidation and de-sulfurization in the molten bath and the loss of theadditional elements during the maintenance of the bath after theaddition should be compensated so that a suflicient quantity to hold thenecessary amount of elements retained in the final casting or steelingot should be added.

This invention can widely be applied to all kinds of steels such ascarbon steels and alloy steels; that is,'it includes ordinary steels'with nodular graphite and alloy steels having nodular graphite togetherwith one or more special elements selected from the group consisting ofsilicon, manganese, nickel, chromium, copper, tungsten, molybdenum,titanium, cobalt and zirconium, and their castings and ingots, which areforgea-ble. The specific physical properties of such steels havingnodular graphite structure show a remarkable improvement in thestrength, a reduction of notch-effect, an increase in damping capacity,a reduction of coefficient of expansion, an increase of abrasion andoxidation resistances, and a reduction of thermal distortion.

Example I A molten bath of hyper-eutectoid steel containing about 1.5%of carbon, about 0.8% of silicon, about 0.4% of manganese, and about0.03% of phosphor was made and to this molten bath was added about 0.3%of calcium, 0.04% of cerium and 0.1% of barium. For the addition ofcalcium, Fe-Ca-Si alloy (33.5% Fe, 29.5% Ca and 35% Si) was used and forcerium misch metal was used and for barium the metallic barium was used.

Such a casting produced by this invention was a hypereutectoid steelhaving very fine nodular graphite structure, and showed a tensilestrength of 65.4 kg./mrn. an elongation of 3.5% (gauge length 50 mm.)and a Brinell hardness number of 241.

Example II A molten bath of hypo-eutectoid steel containing 0.6% ofcarbon, 0.8% of silicon, 0.04% of manganese, and 0.02% of phosphor wasprepared and to this molten bath was added 0.2% of calcium. The calciumwas added as the FeCa-Si alloy (33.5% Fe, 39.5% Ca and 36% Si) mixedwith some calcium fluoride (flux).

A steel ingot produced by this invention is of hypoeutectoid steelconsisting of very fine nodular graphite dispersed in a matrix offerrite and pearlite, and showed a tensile strength of 66.2 kg./mm. anelongation of 14% (gauge length 50 mm.) and a Brinell hardness number of187.

Example III A molten bath of Hadfield steel containing 1.25% of carbon,0.5% of silicon, 13.5% of manganese, and 0.02% of phosphor was preparedand to this bath was added 0.31% of calcium using Fe-Ca-Si alloy (25.0%Fe, 31.6% Ca and 42.7% Si) mixed with 0.4% of calcium fluoride (flux).

A casting prepared by the above method of this invention was a steelhaving very fine nodular graphite and cementite structure, and when itwas quenched in water at a high temperature it showed austenitestructure containing very fine nodular graphite and was stronger thanordinary Hadfield steel, and could be easily machined. The tensilestrength of the hardened cast steel was 54 kg./mm. the elongation 12%(gauge length 50 mm.), the reduction of area 27% and Brinell hardnessnumber of 206.

Example IV A molten bath of chromium-tungsten steel containing 0.7% ofcarbon, 0.5% of silicon, about 0.5% of chromium, about 2% of tungstenand 0.03% of phosphor was made and to this bath was added 0.31% ofcalcium as Fe-Ca-Si alloy (25.7% Fe, 31.6% Ca and 42.7% Si) mixed with0.7% of calcium fluoride as flux and 0.04% of cerium as misch metal.

The forged product made by the above method of this invention was a verystrong chromium-tungsten steel containing fine nodular graphite andcementite, without acicular carbide which usually precipitates in thecrystal grains; the steel showed very good machinability.

What I claim is:

l. A method of manufacturing steel castings, which comprises adding to amolten bath of steel containing less than 1.7% of carbon, 0.2 to 6.0% ofiron-calcium-silicon alloy and 0.005 to 0.15% of cerium so as to obtaina casting containing about 0.001 to 0.5% of calcium and less than 0.15of cerium.

2. A nodular graphite steel containing calcium and cerium in the amountof 0.001 to 0.5 of calcium and 0.005 to 0.15% of cerium and the balancea steel composition with less than about 1.7% of carbon, said castingbeing characterized in the as-cast condition by a microstructurecomprised of soft, gray-colored, substantially spherical graphiteparticles dispersed in ferrite and pearlite.

3. A nodular graphite steel having the characteristic, in the as-castcondition, of a microstructure containing substantially sphericalparticles of uncombined carbon dispersed in ferrite and pearlite andbeing comprised of about 0.001 to 0.5% of calcium with the balance steelcomposition with less than about 1.7% of carbon.

References Cited in the tile of this patent UNITED STATES PATENTS2,280,283 Crafts Apr. 21, 1942 2,488,511 Morrogh Nov. 15, 1949 2,610,912Millis et al Sept. 16, 1952 2,765,225 Carter et al. Oct. 2, 19562,948,605 Ihrig Aug. 9, 1960 FOREIGN PATENTS 774,138 Great Britain May8, 1957 OTHER REFERENCES Carlsson: Jernkontorets Annaler, vol. 137, No.7, 1948,

pages 221-236. Published in Stockholm, Sweden.

De Sy: American Foundryman, February 1951, pages 41-45. Published by theAmerican Foundrymens Society, Chicago, Illinois.

2. A NODULAR GRAPHITE STEEL CONTAINING CALCIUM AND CERIUM IN THE AMOUNTOF 0.001 TO 0.5% OF CALCIUM AND 0.005 TO 0.15% OF CERIUM AND THE BALANCEA STEEL COMPOSITION WITH LESS THAN ABOUT 1.7% OC CARBON, SAID CASTINGBEING CHARACTERIZED IN THE AS-CAST CONDITION BY A MICROSTRUCTURECOMPRISED OF SOFT, GRAY-COLORED, SUBSTANTIALLY SPHERICAL GRAPHITEPARTICLES DISPERSED IN FERRITE AND PEARLITE.
 3. A NODULAR GRAPHITE STEELHAVING THE CHARACTERISTIC, IN THE AS-CAST CONDITION, OF A MICROSTRUCTURECONTAINING SUBSTANTIALLY SPHERICAL PARTICLES OF UNCOMBINED CARBONDISPERSED IN FERRITE AND PEARLITE AND BEING COMPRISED OF ABOUT 0.001 TO0.5% OF CALCIUM WITH THE BALANCE STEEL COMPOSITION WITH LESS THAN ABOUT1.7% OF CARBON.